MONDAY, Aug. 11 (HealthDay News) -- Drugs known as bisphosphonates, commonly prescribed to treat the bone-weakening disease osteoporosis, don't appear to reduce the risk of breast cancer as previously thought, new research finds.

"We found that postmenopausal women who took a bisphosphonate for three or four years did not have a decreased risk in breast cancer," said study author Trisha Hue, an epidemiologist at the University of California, San Francisco.

When prior observational studies noted links between the drugs' use and lower breast cancer risk in, experts assumed the drugs deserved the credit. However, based on the new findings, Hue suspects that low estrogen levels may have been the risk-reducer.

"Women who get bisphosphonates have a low bone mass density," she said. "If you have low bone mass density, you probably have low estrogen." This is because the hormone, which is depleted after menopause, helps regulate bone mass and strength. Many types of breast cancer need estrogen to grow, she said.

The study is published Aug. 11 in JAMA Internal Medicine.

Previous studies that found the fringe benefit of reduced breast cancer risk were observational, meaning people taking the drugs happened to have lower breast cancer risk. But the research didn't establish a direct cause-and-effect relationship.

Hue's team decided to take a look at two randomized clinical trials that compared two popular osteoporosis drugs, Fosamax and Reclast, to see if the women taking the drugs were less likely to get breast cancer than women given a placebo, or sham drug.

In one study, nearly 6,500 women aged 55 to 81 were assigned to take either Fosamax (alendronate) or a placebo. After about four years, similar numbers of women got breast cancer -- 1.8 percent of those on the drug and 1.5 percent of those on placebo.

The second study compared more than 7,700 women, aged 65 to 89, given Reclast (zoledronic acid) intravenously or a placebo IV infusion once a year. After nearly three years of follow-up, less than 1 percent of women in each group developed breast cancer.

One expert said the results underscore the importance of randomized clinical trials in demonstrating cause-and-effect.

"In this case, two randomized studies have disproved the notion that bisphosphonates reduce the risk of beast cancer, a finding that was supported by observational studies," said Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City.

Both types of studies are important, explained Dr. Jill Maura Rabin, a professor of obstetrics and gynecology at Hofstra North Shore-LIJ School of Medicine in New York.

While the new analysis suggests the drugs don't protect against breast cancer, Rabin said follow-up for longer than three or four years might still have found a benefit since breast cancer can take years to develop.

Even so, the new research simply suggests that women prescribed the drugs for bone health should continue taking them, Rabin said, but just not expect a bonus of lower breast cancer risk.